1. Field of the Invention
The present invention relates to a piezoelectric element, a droplet-ejecting head and a droplet-ejecting apparatus, and in particular, to a piezoelectric element, for example, for recording characters and images on a recording medium by ejecting droplets and forming fine patterns or thin films on a substrate, a droplet-ejecting head using the piezoelectric element, and a droplet-ejecting apparatus using the droplet-ejecting head.
2. Description of the Related Art
Droplet-ejecting heads, which generate pressure wave (acoustic wave) in liquid filled in a pressure-generating chamber by using a pressure-generating means such as a piezoelectric actuator and eject droplets through a nozzle connected to the pressure-generating chamber (or pressure chamber) by the pressure wave, are generally known well. Such a piezoelectric actuator generally consists of a piezoelectric element that deforms according to image information and a vibration plate that vibrates by deformation of the piezoelectric element and expands or compresses the pressure-generating chamber.
To ensure long-term reliability of the piezoelectric actuator, it is necessary to overcome problems such as troubles of the piezoelectric element during operation, especially under highly humid environment (in particular, deterioration of capacity caused by generation of oxygen holes and elution of lead ion) and defects in electrode film (electromigration), and thus, electrodes in various configurations were studied.
For example, known is a case a method of using a base metal as the electrode in a piezoelectric element (e.g., JP-A No. 2004-152911). The invention of JP-A No. 2004-152911 prevents reducing reaction of piezoelectric elements by forming top and bottom electrodes with different metals.
Also known is a method of forming an electrode layer with a composition resistant to the reducing action due to moisture (e.g., JP-A No. 10-264384). The invention of JP-A No. 10-264384 suppresses electrolysis of water and thus prevents the reducing reaction of piezoelectric member by forming two electrode layers made of different metals on a piezoelectric element and regulating the relationship between the standard electrode potentials thereof.
Also known is a method of forming a conductive metal oxide layer on the positive side electrode of a piezoelectric element and additionally a metal layer for solder junction thereon (e.g., JP-A No. 2001-88296). The invention of JP-A No. 2001-88296, which uses a metal which can form solder junction as the principal metal in the positive electrode and forms a layer of an alloy containing, for example, a noble metal for prevention of Pb-ion elution, allows reduction in the thickness of metal layer, and is thus, advantageous from the points of productivity and cost reduction.
In addition, a method of using a metal having a standard electrode potential of 0 V or more as the high-electric potential electrode and a metal oxide as the low-electric potential electrode is also known (e.g., JP-A No. 2005-32955). The invention of JP-A No. 2005-32955 suppresses electrolysis of water and prevents reducing reaction of the piezoelectric element, by regulating the relationship between the standard electrode potentials of the top and bottom electrodes. It is also possible to obtain favorable electrical properties (in particular, conductivity) reliably by forming a layer other than metal oxide layer as the low-electric potential electrode.
However, the conventional inventions described above could not prevent deterioration of the piezoelectric element over time sufficiently when it is used under high-temperature and high-humidity environment.